Human Metapneumovirus Is Capable of Entering Cells by Fusion with Endosomal Membranes

Advances in the Relationship between Respiratory Viruses and Asthma

Several studies have reported that viral infection is closely associated with the onset, progression, and exacerbation of asthma. The purpose of this review is to summarize the role that viral infections have in the pathogenesis of asthma onset and exacerbations, as well as discuss interrelated protective and risk factors of asthma and current treatment options. Furthermore, we present current knowledge of the innate immunological pathways driving host defense, including changes in the epithelial barrier. In addition, we highlight the importance of the genetics and epigenetics of asthma and virus susceptibility. Moreover, the involvement of virus etiology from bronchiolitis and childhood wheezing to asthma is described. The characterization and mechanisms of action of the respiratory viruses most frequently related to asthma are mentioned.

Hemagglutinin stability as a key determinant of influenza A virus transmission via air

To cause pandemics, zoonotic respiratory viruses need to adapt to replication in and spread between humans, either via (indirect or direct) contact or through the air via droplets and aerosols. To render influenza A viruses transmissible via air, three phenotypic viral properties must change, of which receptor-binding specificity and polymerase activity have been well studied. However, the third adaptive property, hemagglutinin (HA) acid stability, is less understood. Recent studies show that there may be a correlation between HA acid stability and virus survival in the air, suggesting that a premature conformational change of HA, triggered by low pH in the airways or droplets, may render viruses noninfectious before they can reach a new host. We here summarize available data from (animal) studies on the impact of HA acid stability on airborne transmission and hypothesize that the transmissibility of other respiratory viruses may also be impacted by an acidic environment in the airways.

Antiviral strategies against human metapneumovirus: Targeting the fusion protein

Human metapneumoviruses have emerged in the past decades as an important global pathogen that causes severe upper and lower respiratory tract infections. Children under the age of 2, the elderly and immunocompromised individuals are more susceptible to HMPV infection than the general population due to their suboptimal immune system. Despite the recent discovery of HMPV as a novel important respiratory virus, reports have rapidly described its epidemiology, biology, and pathogenesis. However, progress is still to be made in the development of vaccines and drugs against HMPV infection as none are currently available. Herein, we discuss the importance of HMPV and review the reported strategies for anti-HMPV drug candidates. We also present the fusion protein as a promising antiviral drug target due to its multiple roles in the HMPV lifecycle. This key viral protein has previously been targeted by a range of inhibitors, which will be discussed as they represent opportunities for future drug design.

New Insights into the Host-Pathogen Interaction of Mycoplasma gallisepticum and Avian Metapneumovirus in Tracheal Organ Cultures of Chicken

Respiratory pathogens are a health threat for poultry. Co-infections lead to the exacerbation of clinical symptoms and lesions. Mycoplasma gallisepticum (M. gallispeticum) and Avian Metapneumovirus (AMPV) are two avian respiratory pathogens that co-circulate worldwide. The knowledge about the host-pathogen interaction of M. gallispeticum and AMPV in the chicken respiratory tract is limited. We aimed to investigate how co-infections affect the pathogenesis of the respiratory disease and whether the order of invading pathogens leads to changes in host-pathogen interaction. We used chicken tracheal organ cultures (TOC) to investigate pathogen invasion and replication, lesion development, and selected innate immune responses, such as interferon (IFN) α, inducible nitric oxide synthase (iNOS) and IFNλ mRNA expression levels. We performed mono-inoculations (AMPV or M. gallispeticum) or dual-inoculations in two orders with a 24-h interval between the first and second pathogen. Dual-inoculations compared to mono-inoculations resulted in more severe host reactions. Pre-infection with AMPV followed by M. gallispeticum resulted in prolonged viral replication, more significant innate immune responses, and lesions (p < 0.05). AMPV as the secondary pathogen impaired the bacterial attachment process. Consequently, the M. gallispeticum replication was delayed, the innate immune response was less pronounced, and lesions appeared later. Our results suggest a competing process in co-infections and offer new insights in disease processes.

Newcastle Disease Virus Entry into Chicken Macrophages via a pH-Dependent, Dynamin and Caveola-Mediated Endocytic Pathway That Requires Rab5

The cellular entry pathways and the mechanisms of Newcastle disease virus (NDV) entry into cells are poorly characterized. In this study, we demonstrated that chicken interferon-induced transmembrane protein 1 (chIFITM1), which is located in the early endosomes, could limit the replication of NDV in chicken macrophage cell line HD11, suggesting the endocytic entry of NDV into chicken macrophages. Then, we presented a systematic study about the entry mechanism of NDV into chicken macrophages. First, we demonstrated that a low-pH condition and dynamin were required during NDV entry. However, NDV entry into chicken macrophages was independent of clathrin-mediated endocytosis. We also found that NDV entry was dependent on membrane cholesterol. The NDV entry and replication were significantly reduced by nystatin and phorbol 12-myristate 13-acetate treatment, overexpression of dominant-negative (DN) caveolin-1, or knockdown of caveolin-1, suggesting that NDV entry depends on caveola-mediated endocytosis. However, macropinocytosis did not play a role in NDV entry into chicken macrophages. In addition, we found that Rab5, rather than Rab7, was involved in the entry and traffic of NDV. The colocalization of NDV with Rab5 and early endosome suggested that NDV virion was transported to early endosomes in a Rab5-dependent manner after internalization. Of particular note, the caveola-mediated endocytosis was also utilized by NDV to enter primary chicken macrophages. Moreover, NDV entered different cell types using different pathways. Collectively, our findings demonstrate for the first time that NDV virion enters chicken macrophages via a pH-dependent, dynamin and caveola-mediated endocytosis pathway and that Rab5 is involved in the traffic and location of NDV. IMPORTANCE Although the pathogenesis of Newcastle disease virus (NDV) has been extensively studied, the detailed mechanism of NDV entry into host cells is largely unknown. Macrophages are the first-line defenders of host defense against infection of pathogens. Chicken macrophages are considered one of the main types of target cells during NDV infection. Here, we comprehensively investigated the entry mechanism of NDV in chicken macrophages. This is the first report to demonstrate that NDV enters chicken macrophages via a pH-dependent, dynamin and caveola-mediated endocytosis pathway that requires Rab5. The result is important for our understanding of the entry of NDV in chicken macrophages, which will further advance the knowledge of NDV pathogenesis and provide useful clues for the development of novel preventive or therapeutic strategies against NDV infection. In addition, this information will contribute to our further understanding of pathogenesis with regard to other members of the Avulavirus genus in the Paramyxoviridae family.

Host Components That Modulate the Disease Caused by hMPV

Human metapneumovirus (hMPV) is one of the main pathogens responsible for acute respiratory infections in children up to 5 years of age, contributing substantially to health burden. The worldwide economic and social impact of this virus is significant and must be addressed. The structural components of hMPV (either proteins or genetic material) can be detected by several receptors expressed by host cells through the engagement of pattern recognition receptors. The recognition of the structural components of hMPV can promote the signaling of the immune response to clear the infection, leading to the activation of several pathways, such as those related to the interferon response. Even so, several intrinsic factors are capable of modulating the immune response or directly inhibiting the replication of hMPV. This article will discuss the current knowledge regarding the innate and adaptive immune response during hMPV infections. Accordingly, the host intrinsic components capable of modulating the immune response and the elements capable of restricting viral replication during hMPV infections will be examined.

Mosquito Defensins Enhance Japanese Encephalitis Virus Infection by Facilitating Virus Adsorption and Entry within the Mosquito

Japanese encephalitis virus (JEV) is a viral zoonosis that can cause viral encephalitis, death, and disability. Although the Culex mosquito is the primary vector of JEV, little is known about JEV transmission by this kind of mosquito. Here, we found that mosquito defensin facilitated the adsorption of JEV on target cells via the defensin/lipoprotein receptor-related protein 2 (LRP2) axis. Mosquito defensin bound the ED III domain of the viral envelope (E) protein and directly mediated efficient virus adsorption on the target cell surface; the receptor LRP2, which is expressed on the cell surface, affected defensin-dependent adsorption. As a result, mosquito defensin enhanced JEV infection in the salivary gland, increasing the possibility of viral transmission by mosquitoes. These findings demonstrate the novel role of mosquito defensin in JEV infection and the mechanisms through which the virus exploits mosquito defensin for infection and transmission.IMPORTANCE In this study, we observed the complex roles of mosquito defensin in JEV infection; mosquito defensin exhibited a weak antiviral effect but strongly enhanced binding. In the latter, defensin directly binds the ED III domain of the viral E protein and promotes the adsorption of JEV to target cells by interacting with lipoprotein receptor-related protein 2 (LRP2), thus accelerating virus entry. Together, our results indicate that mosquito defensin plays an important role in facilitating JEV infection and potential transmission.

Antibody recognition of the Pneumovirus fusion protein trimer interface

Human metapneumovirus (hMPV) is a leading cause of viral respiratory infection in children, and can cause severe lower respiratory tract infection in infants, the elderly, and immunocompromised patients. However, there remain no licensed vaccines or specific treatments for hMPV infection. Although the hMPV fusion (F) protein is the sole target of neutralizing antibodies, the immunological properties of hMPV F remain poorly understood. To further define the humoral immune response to the hMPV F protein, we isolated two new human monoclonal antibodies (mAbs), MPV458 and MPV465. Both mAbs are neutralizing in vitro and were determined to target a unique antigenic site using competitive biolayer interferometry. We determined both MPV458 and MPV465 have higher affinity for monomeric hMPV F than trimeric hMPV F. MPV458 was co-crystallized with hMPV F, and the mAb primarily interacts with an alpha helix on the F2 region of the hMPV F protein. Surprisingly, the major epitope for MPV458 lies within the trimeric interface of the hMPV F protein, suggesting significant breathing of the hMPV F protein must occur for host immune recognition of the novel epitope. In addition, significant glycan interactions were observed with a somatically mutated light chain framework residue. The data presented identifies a novel epitope on the hMPV F protein for epitope-based vaccine design, and illustrates a new mechanism for human antibody neutralization of viral glycoproteins.

Respiratory Syncytial Virus and Human Metapneumovirus Infections in Three-Dimensional Human Airway Tissues Expose an Interesting Dichotomy in Viral Replication, Spread, and Inhibition by Neutralizing Antibodies

Respiratory syncytial virus (RSV) and human metapneumovirus (HMPV) are two of the leading causes of respiratory infections in children and elderly and immunocompromised patients worldwide. There is no approved treatment for HMPV and only one prophylactic treatment against RSV, palivizumab, for high-risk infants. Better understanding of the viral lifecycles in a more relevant model system may help identify novel therapeutic targets. By utilizing three-dimensional (3-D) human airway tissues to examine viral infection in a physiologically relevant model system, we showed that RSV infects and spreads more efficiently than HMPV, with the latter requiring higher multiplicities of infection (MOIs) to yield similar levels of infection. Apical ciliated cells were the target for both viruses, but RSV apical release was significantly more efficient than HMPV. In RSV- or HMPV-infected cells, cytosolic inclusion bodies containing the nucleoprotein, phosphoprotein, and respective viral genomic RNA were clearly observed in human airway epithelial (HAE) culture. In HMPV-infected cells, actin-based filamentous extensions were more common (35.8%) than those found in RSV-infected cells (4.4%). Interestingly, neither RSV nor HMPV formed syncytia in HAE tissues. Palivizumab and nirsevimab effectively inhibited entry and spread of RSV in HAE tissues, with nirsevimab displaying significantly higher potency than palivizumab. In contrast, 54G10 completely inhibited HMPV entry but only modestly reduced viral spread, suggesting HMPV may use alternative mechanisms for spread. These results represent the first comparative analysis of infection by the two pneumoviruses in a physiologically relevant model, demonstrating an interesting dichotomy in the mechanisms of infection, spread, and consequent inhibition of the viral lifecycles by neutralizing monoclonal antibodies.IMPORTANCE Respiratory syncytial virus and human metapneumovirus are leading causes of respiratory illness worldwide, but limited treatment options are available. To better target these viruses, we examined key aspects of the viral life cycle in three-dimensional (3-D) human airway tissues. Both viruses establish efficient infection through the apical surface, but efficient spread and apical release were seen for respiratory syncytial virus (RSV) but not human metapneumovirus (HMPV). Both viruses form inclusion bodies, minimally composed of nucleoprotein (N), phosphoprotein (P), and viral RNA (vRNA), indicating that these structures are critical for replication in this more physiological model. HMPV formed significantly more long, filamentous actin-based extensions in human airway epithelial (HAE) tissues than RSV, suggesting HMPV may promote cell-to-cell spread via these extensions. Lastly, RSV entry and spread were fully inhibited by neutralizing antibodies palivizumab and the novel nirsevimab. In contrast, while HMPV entry was fully inhibited by 54G10, a neutralizing antibody, spread was only modestly reduced, further supporting a cell-to-cell spread mechanism.

Metapneumovirus humano: epidemiología y posibles tratamientos profilácticos

In 2001 in the Netherlands, Human metapneumovirus (hMPV) was identified as a “new” etiologic agent causing acute respiratory infections in children younger than 5 years old; however, it has also been isolated in the elderly and immunocompromised people. This virus is considered the second etiological agent in acute diseases of the respiratory tract. Currently, the estimated cost of IRAs in our country is of 9,000USD per inpatient. hMPV is a member of the genus Metapneumovirus, family Pneumoviridae, and it belongs to the order Mononegavirales that is part of the negative single-stranded ribonucleic acid (RNA) virus, consisting of eight genes ordered: 3’-N-P-M-FM2-SH-G-L-5 ‘, and which encodes for 9 proteins. Of these proteins, the F fusion glycoprotein is highly conserved in the genus Metapneumovirus, and is the major antigenic determinant, and because an approved vaccine doesn’t exist, it has been used as a candidate epitope for the design of a vaccine that confers host immunity or as a therapeutic target in the creation of antiviral peptides that inhibit the fusion of the virus to its target cell and to avoid infection in subjects at high risk of contagion since there is currently none accepted by COFEPRIS as a prophylactic treatment against hMPV. Key words: hMPV; respiratory infections; epitopes; protein F;vaccines.

Human Metapneumovirus: A Largely Unrecognized Threat to Human Health

Human metapneumovirus (HMPV) infects most children by five years of age. The virus can cause both upper and lower respiratory tract disease and can be life threatening. High-risk populations include young children who are exposed to virus for the first time and the elderly. Currently, there is no standard treatment nor licensed vaccine for HMPV, although several attractive vaccine candidates have been developed for pre-clinical studies. A raised awareness of the impact of HMPV on public health is needed to drive research, complete vaccine development, and thereby prevent significant virus-associated morbidities and mortalities worldwide.

A Potent Neutralizing Site III-Specific Human Antibody Neutralizes Human Metapneumovirus In Vivo

Human metapneumovirus (hMPV) is a leading cause of viral lower respiratory tract infection in children. The sole target of neutralizing antibodies targeting hMPV is the fusion (F) protein, a class I viral fusion protein mediating virus-cell membrane fusion. There have been several monoclonal antibodies (mAbs) isolated that neutralize hMPV; however, determining the antigenic sites on the hMPV F protein mediating such neutralizing antibody generation would assist efforts for effective vaccine design. In this report, the isolation and characterization of four new human mAbs, termed MPV196, MPV201, MPV314, and MPV364, are described. Among the four mAbs, MPV364 was found to be the most potent neutralizing mAb in vitro Binding studies with monomeric and trimeric hMPV F revealed that MPV364 had the weakest binding affinity for monomeric hMPV F compared to the other three mAbs, yet binding experiments with trimeric hMPV F showed limited differences in binding affinity, suggesting that MPV364 targets an antigenic site incorporating two protomers. Epitope binning studies showed that MPV364 targets antigenic site III on the hMPV F protein and competes for binding with previously discovered mAbs MPE8 and 25P13, both of which cross-react with the respiratory syncytial virus (RSV) F protein. However, MPV364 does not cross-react with the RSV F protein, and the competition profile suggests that it binds to the hMPV F protein in a binding pose slightly shifted from mAbs MPE8 and 25P13. MPV364 was further assessed in vivo and was shown to substantially reduce viral replication in the lungs of BALB/c mice. Overall, these data reveal a new binding region near antigenic site III of the hMPV F protein that elicits potent neutralizing hMPV F-specific mAbs and provide a new panel of neutralizing mAbs that are candidates for therapeutic development.IMPORTANCE Recent progress in understanding the human immune response to respiratory syncytial virus has paved the way for new vaccine antigens and therapeutics to prevent and treat disease. Progress toward understanding the immune response to human metapneumovirus (hMPV) has lagged behind, although hMPV is a leading cause of lower respiratory tract infection in children. In this report, we advanced the field by isolating a panel of human mAbs to the hMPV F protein. One potent neutralizing mAb, MPV364, targets antigenic site III on the hMPV F protein and incorporates two protomers into its epitope yet is unique from previously discovered site III mAbs, as it does not cross-react with the RSV F protein. We further examined MPV364 in vivo and found that it limits viral replication in BALB/c mice. Altogether, these data provide new mAb candidates for therapeutic development and provide insights into hMPV vaccine development.

IFITM3 Restricts Human Metapneumovirus Infection

Human metapneumovirus (hMPV) utilizes a bifurcated cellular entry strategy, fusing either with the plasma membrane or, after endocytosis, with the endosome membrane. Whether cellular factors restrict or enhance either entry pathway is largely unknown. We found that the interferon-induced transmembrane protein 3 (IFITM3) inhibits hMPV infection to an extent similar to endocytosis-inhibiting drugs, and an IFITM3 variant that accumulates at the plasma membrane in addition to its endosome localization provided increased virus restriction. Mechanistically, IFITM3 blocks hMPV F protein-mediated membrane fusion, and inhibition of infection was reversed by the membrane destabilizing drug amphotericin B. Conversely, we found that infection by some hMPV strains is enhanced by the endosomal protein toll-like receptor 7 (TLR7), and that IFITM3 retains the ability to restrict hMPV infection even in cells expressing TLR7. Overall, our results identify IFITM3 as an endosomal restriction factor that limits hMPV infection of cells.

A medium-throughput screen for inhibitors of human metapneumovirus

Human metapneumovirus, a paramyxovirus discovered in 2001, is a major cause of lower respiratory infection in adults and children worldwide. There are no licensed vaccines or drugs for human metapneumovirus. We developed a fluorescent, cell-based medium-throughput screening assay for human metapneumovirus that captures inhibitors of all stages of the viral lifecycle except budding of progeny virus particles from the cell membrane. We optimized and validated the assay and performed a successful medium-throughput screening. A number of hits were identified, several of which were confirmed to inhibit viral replication in secondary assays. This assay offers potential to discover new antivirals for human metapneumovirus and related respiratory viruses. Compounds discovered using the medium-throughput screening may also provide useful probes of viral biology.

Consensus and variations in cell line specificity among human metapneumovirus strains

Human metapneumovirus (HMPV) has been a notable etiological agent of acute respiratory infection in humans, but it was not discovered until 2001, because HMPV replicates only in a limited number of cell lines and the cytopathic effect (CPE) is often mild. To promote the study of HMPV, several groups have generated green fluorescent protein (GFP)-expressing recombinant HMPV strains (HMPV^GFP). However, the growing evidence has complicated the understanding of cell line specificity of HMPV, because it seems to vary notably among HMPV strains. In addition, unique A2b clade HMPV strains with a 180-nucleotide duplication in the G gene (HMPV A2b_180nt-dup strains) have recently been detected. In this study, we re-evaluated and compared the cell line specificity of clinical isolates of HMPV strains, including the novel HMPV A2b_180nt-dup strains, and six recombinant HMPV^GFP strains, including the newly generated recombinant HMPV A2b_180nt-dup strain, MG0256-EGFP. Our data demonstrate that VeroE6 and LLC-MK2 cells generally showed the highest infectivity with any clinical isolates and recombinant HMPV^GFP strains. Other human-derived cell lines (BEAS-2B, A549, HEK293, MNT-1, and HeLa cells) showed certain levels of infectivity with HMPV, but these were significantly lower than those of VeroE6 and LLC-MK2 cells. Also, the infectivity in these suboptimal cell lines varied greatly among HMPV strains. The variations were not directly related to HMPV genotypes, cell lines used for isolation and propagation, specific genome mutations, or nucleotide duplications in the G gene. Thus, these variations in suboptimal cell lines are likely intrinsic to particular HMPV strains.

Human metapneumovirus fusion protein triggering: Increasing complexities by analysis of new HMPV fusion proteins

The human metapneumovirus (HMPV) fusion protein (F) mediates fusion of the viral envelope and cellular membranes to establish infection. HMPV F from some, but not all, viral strains promotes fusion only after exposure to low pH. Previous studies have identified several key residues involved in low pH triggering, including H435 and a proposed requirement for glycine at position 294. We analyzed the different levels of fusion activity, protein expression and cleavage of three HMPV F proteins not previously examined. Interestingly, low pH-triggered fusion in the absence of G294 was identified in one F protein, while a novel histidine residue (H434) was identified that enhanced low pH promoted fusion in another. The third F protein failed to promote cell-to-cell fusion, suggesting other requirements for F protein triggering. Our results demonstrate HMPV F triggering is more complex than previously described and suggest a more intricate mechanism for fusion protein function and activation.

The role of lipid rafts in cell entry of human metapneumovirus

Human metapneumovirus (hMPV) is a crucial pathogen in children. A cell entry is the first step for infection. Our previous study indicated that there was an endocytosis pathway for hMPV cell entry. Lipid raft is a specific structure at the cell surface and it has been demonstrated to play an important role in endocytosis process of many viruses. In this study, we investigated whether and how lipid raft can take part in the hMPV entry. The confocal microscope was used to detect colocalization of hMPV and lipid raft marker. We demonstrated that colocalizations were increased along with the viral infection and hMPV particles transferred to the perinuclear region with lipid raft. When specific lipid raft inhibitors: methyl‐β cyclodextrin and nystatin were used, hMPV cell entry was inhibited and viral titer decreased dramatically. With the replenishment of exogenous cholesterol, hMPV recovered quickly. These data suggest that lipid raft plays an important role in hMPV endocytosis and maybe one of the pathways for hMPV cell entry.

This study showed lipid raft, as the specific structure at cell surface, plays an important role in hMPV endocytosis and maybe the one of the pathways for hMPV cell entry.

This study gave a better understanding of the mechanisms of hMPV cell entry and a new way to prevent and treat its infection.

Development and optimization of a direct plaque assay for trypsin-dependent human metapneumovirus strains

Human metapneumovirus (HMPV) is a non-segmented, negative strand RNA virus belonging to the family Pneumoviridae, previously a subfamily of Paramyxoviridae. It is a leading cause of lower respiratory tract infection in infants, children, and adults with underlying medical conditions. HMPV grows poorly in cell culture and requires trypsin to cleave and mature the virus particles, which adds to the challenge of HMPV research. Currently, an indirect immuno-staining assay is commonly used to titrate HMPV, which is time-consuming and costly. In order to simplify virus quantification for HMPV, a direct plaque assay was developed. By optimizing trypsin concentration and other supplements in the agarose overlay, it was found that HMPV strains from all four subgroups formed clear and countable plaques 5-7 days post-infection. Animal tissue homogenate can also be directly titrated with this assay. Compared with the traditional assay, the direct plaque assay yields similar titer result, but saves time and eliminates the use of antibodies. Potentially, it can also be applied to plaque purification for HMPV clinical isolates. The direct plaque assay will be a valuable tool in HMPV research.

Early events during human coronavirus OC43 entry to the cell

The Coronaviridae family clusters a number of large RNA viruses, which share several structural and functional features. However, members of this family recognize different cellular receptors and exploit different entry routes, what affects their species specificity and virulence. The aim of this study was to determine how human coronavirus OC43 enters the susceptible cell. Using confocal microscopy and molecular biology tools we visualized early events during infection. We found that the virus employs caveolin-1 dependent endocytosis for the entry and the scission of virus-containing vesicles from the cell surface is dynamin-dependent. Furthermore, the vesicle internalization process requires actin cytoskeleton rearrangements. With our research we strove to broaden the understanding of the infection process, which in future may be beneficial for the development of a potential therapeutics.

Human metapneumovirus - what we know now

Human metapneumovirus (HMPV) is a leading cause of acute respiratory infection, particularly in children, immunocompromised patients, and the elderly. HMPV, which is closely related to avian metapneumovirus subtype C, has circulated for at least 65 years, and nearly every child will be infected with HMPV by the age of 5. However, immunity is incomplete, and re-infections occur throughout adult life. Symptoms are similar to those of other respiratory viral infections, ranging from mild (cough, rhinorrhea, and fever) to more severe (bronchiolitis and pneumonia). The preferred method for diagnosis is reverse transcription-polymerase chain reaction as HMPV is difficult to culture. Although there have been many advances made in the past 16 years since its discovery, there are still no US Food and Drug Administration-approved antivirals or vaccines available to treat HMPV. Both small animal and non-human primate models have been established for the study of HMPV. This review will focus on the epidemiology, transmission, and clinical manifestations in humans as well as the animal models of HMPV pathogenesis and host immune response.

Human Metapneumovirus Induces Formation of Inclusion Bodies for Efficient Genome Replication and Transcription

Human metapneumovirus (HMPV) causes significant upper and lower respiratory disease in all age groups worldwide. The virus possesses a negative-sense single-stranded RNA genome of approximately 13.3 kb encapsidated by multiple copies of the nucleoprotein (N), giving rise to helical nucleocapsids. In addition, copies of the phosphoprotein (P) and the large RNA polymerase (L) decorate the viral nucleocapsids. After viral attachment, endocytosis, and fusion mediated by the viral glycoproteins, HMPV nucleocapsids are released into the cell cytoplasm. To visualize the subsequent steps of genome transcription and replication, a fluorescence in situ hybridization (FISH) protocol was established to detect different viral RNA subpopulations in infected cells. The FISH probes were specific for detection of HMPV positive-sense RNA (+RNA) and viral genomic RNA (vRNA). Time course analysis of human bronchial epithelial BEAS-2B cells infected with HMPV revealed the formation of inclusion bodies (IBs) from early times postinfection. HMPV IBs were shown to be cytoplasmic sites of active transcription and replication, with the translation of viral proteins being closely associated. Inclusion body formation was consistent with an actin-dependent coalescence of multiple early replicative sites. Time course quantitative reverse transcription-PCR analysis suggested that the coalescence of inclusion bodies is a strategy to efficiently replicate and transcribe the viral genome. These results provide a better understanding of the steps following HMPV entry and have important clinical implications.IMPORTANCE Human metapneumovirus (HMPV) is a recently discovered pathogen that affects human populations of all ages worldwide. Reinfections are common throughout life, but no vaccines or antiviral treatments are currently available. In this work, a spatiotemporal analysis of HMPV replication and transcription in bronchial epithelial cell-derived immortal cells was performed. HMPV was shown to induce the formation of large cytoplasmic granules, named inclusion bodies, for genome replication and transcription. Unlike other cytoplasmic structures, such as stress granules and processing bodies, inclusion bodies are exclusively present in infected cells and contain HMPV RNA and proteins to more efficiently transcribe and replicate the viral genome. Though inclusion body formation is nuanced, it corresponds to a more generalized strategy used by different viruses, including filoviruses and rhabdoviruses, for genome transcription and replication. Thus, an understanding of inclusion body formation is crucial for the discovery of innovative therapeutic targets.

Plant-made virus-like particle vaccines bearing the hemagglutinin of either seasonal (H1) or avian (H5) influenza have distinct patterns of interaction with human immune cells in vitro

INTRODUCTION

The recent emergence of avian influenza strains has fuelled concern about pandemic preparedness since vaccines targeting these viruses are often poorly immunogenic. Weak antibody responses to vaccines have been seen across multiple platforms including plant-made VLPs. To better understand these differences, we compared the in vitro responses of human immune cells exposed to plant-made virus-like particle (VLP) vaccines targeting H1N1 (H1-VLP) and H5N1 (H5-VLP).

METHODS

Peripheral blood mononuclear cells (PBMC) from healthy adults were stimulated ex vivo with 2-5µg/mL VLPs bearing the hemagglutinin (HA) of either H1N1 (A/California/7/2009) or H5N1 (A/Indonesia/5/05). VLP-immune cell interactions were characterized by confocal microscopy and flow cytometry 30min after stimulation with dialkylaminostyryl dye-labeled (DiD) VLP. Expression of CD69 and pro-inflammatory cytokines were used to assess innate immune activation 6h after stimulation.

RESULTS

H1- and H5-VLPs rapidly associated with all subsets of human PBMC but exhibited unique binding preferences and frequencies. The H1-VLP bound to 88.7±1.6% of the CD19⁺ B cells compared to only 21.9±1.8% bound by the H5-VLP. At 6h in culture, CD69 expression on B cells was increased in response to H1-VLP but not H5-VLP (22.79±3.42% vs. 6.15±0.82% respectively: p<0.0001). Both VLPs were rapidly internalized by CD14⁺ monocytes resulting in the induction of pro-inflammatory cytokines (i.e.: IL-8, IL-1β, TNFα and IL-6). However, a higher concentration of the H5-VLP was required to induce a comparable response and the pattern of cytokine production differed between VLP vaccines.

CONCLUSIONS

Plant-made VLP vaccines bearing H1 or H5 rapidly elicit immune activation and cytokine production in human PBMC. Differences in the VLP-immune cell interactions suggest that features of the HA proteins themselves, such as receptor specificity, influence innate immune responses. Although not generally considered for inactivated vaccines, the distribution and characteristics of influenza receptor(s) on the immune cells themselves may contribute to both the strength and pattern of the immune response generated.

Structure and immunogenicity of pre-fusion-stabilized human metapneumovirus F glycoprotein

Human metapneumovirus (hMPV) is a frequent cause of bronchiolitis in young children. Its F glycoprotein mediates virus-cell membrane fusion and is the primary target of neutralizing antibodies. The inability to produce recombinant hMPV F glycoprotein in the metastable pre-fusion conformation has hindered structural and immunological studies. Here, we engineer a pre-fusion-stabilized hMPV F ectodomain and determine its crystal structure to 2.6 Å resolution. This structure reveals molecular determinants of strain-dependent acid-induced fusion, as well as insights into refolding from pre- to post-fusion conformations. A dense glycan shield at the apex of pre-fusion hMPV F suggests that antibodies against this site may not be elicited by host immune responses, which is confirmed by depletion studies of human immunoglobulins and by mouse immunizations. This is a major difference with pre-fusion F from human respiratory syncytial virus (hRSV), and collectively our results should facilitate development of effective hMPV vaccine candidates.

Modulation of Host Immunity by the Human Metapneumovirus

Globally, as a leading agent of acute respiratory tract infections in children <5 years of age and the elderly, the human metapneumovirus (HMPV) has gained considerable attention. As inferred from studies comparing vaccinated and experimentally infected mice, the acquired immune response elicited by this pathogen fails to efficiently clear the virus from the airways, which leads to an exaggerated inflammatory response and lung damage. Furthermore, after disease resolution, there is a poor development of T and B cell immunological memory, which is believed to promote reinfections and viral spread in the community. In this article, we discuss the molecular mechanisms that shape the interactions of HMPV with host tissues that lead to pulmonary pathology and to the development of adaptive immunity that fails to protect against natural infections by this virus.

180-Nucleotide Duplication in the G Gene of Human metapneumovirus A2b Subgroup Strains Circulating in Yokohama City, Japan, since 2014

Human metapneumovirus (HMPV), a member of the family Paramyxoviridae, was first isolated in 2001. Seroepidemiological studies have shown that HMPV has been a major etiological agent of acute respiratory infections in humans for more than 50 years. Molecular epidemiological, genetic, and antigenetic evolutionary studies of HMPV will strengthen our understanding of the epidemic behavior of the virus and provide valuable insight for the control of HMPV and the development of vaccines and antiviral drugs against HMPV infection. In this study, the nucleotide sequence of and genetic variations in the G gene were analyzed in HMPV strains prevalent in Yokohama City, in the Kanto area, Japan, between January 2013 and June 2016. As a part of the National Epidemiological Surveillance of Infectious Diseases, Japan, 1308 clinical specimens (throat swabs, nasal swabs, nasal secretions, and nasal aspirate fluids) collected at 24 hospitals or clinics in Yokohama City were screened for 15 major respiratory viruses with a multiplex reverse transcription–PCR assay. HMPV was detected in 91 specimens, accounting for 7.0% of the total specimens, and the nucleotide sequences of the G genes of 84 HMPV strains were determined. Among these 84 strains, 6, 43, 10, and 25 strains were classified into subgroups A2a, A2b, B1, and B2, respectively. Approximately half the HMPV A2b subgroup strains detected since 2014 had a 180-nucleotide duplication (180nt-dup) in the G gene and clustered on a phylogenic tree with four classical 180nt-dup-lacking HMPV A2b strains prevalent between 2014 and 2015. The 180nt-dup causes a 60-amino-acid duplication (60aa-dup) in the G protein, creating 23–25 additional potential acceptor sites for O-linked sugars. Our data suggest that 180nt-dup occurred between 2011 and 2013 and that HMPV A2b strains with 180nt-dup (A2b_180nt-dup HMPV) became major epidemic strains within 3 years. The detailed mechanism by which the A2b_180nt-dup HMPV strains gained an advantage that allowed their efficient spread in the community and the effects of 60aa-dup on HMPV virulence must be clarified.

Current developments and prospects on human metapneumovirus vaccines

INTRODUCTION

Human metapneumovirus (hMPV) has become one of the major pathogens causing acute respiratory infections (ARI) mainly affecting young children, immunocompromised patients, and the elderly. Currently there are no licensed vaccines against this virus. Areas covered: Since the discovery of hMPV in 2001, many groups have focused on developing vaccines against this pathogen. This review presents the outcomes and perspectives derived from preclinical studies performed in cell cultures and animals as well as the only candidate that has reached evaluation in a clinical trial. Limitations of the current vaccine candidates are discussed and perspectives for the development of plant-based vaccines are analyzed. Expert commentary: Several hMPV vaccine candidates are under development with the potential to progress into clinical trials. In parallel, the molecular farming field offers new opportunities to generate innovative vaccines that will offer several advantages in the fight against hMPV.

Inhibition of Human Metapneumovirus Binding to Heparan Sulfate Blocks Infection in Human Lung Cells and Airway Tissues

Human metapneumovirus (HMPV), a recently discovered paramyxovirus, infects nearly 100% of the world population and causes severe respiratory disease in infants, the elderly, and immunocompromised patients. We previously showed that HMPV binds heparan sulfate proteoglycans (HSPGs) and that HMPV binding requires only the viral fusion (F) protein. To characterize the features of this interaction critical for HMPV binding and the role of this interaction in infection in relevant models, we utilized sulfated polysaccharides, heparan sulfate mimetics, and occluding compounds. Iota-carrageenan demonstrated potent anti-HMPV activity by inhibiting binding to lung cells mediated by the F protein. Furthermore, analysis of a minilibrary of variably sulfated derivatives of Escherichia coli K5 polysaccharide mimicking the HS structure revealed that the highly O-sulfated K5 polysaccharides inhibited HMPV infection, identifying a potential feature of HS critical for HMPV binding. The peptide dendrimer SB105-A10, which binds HS, reduced binding and infection in an F-dependent manner, suggesting that occlusion of HS at the target cell surface is sufficient to prevent infection. HMPV infection was also inhibited by these compounds during apical infection of polarized airway tissues, suggesting that these interactions take place during HMPV infection in a physiologically relevant model. These results reveal key features of the interaction between HMPV and HS, supporting the hypothesis that apical HS in the airway serves as a binding factor during infection, and HS modulating compounds may serve as a platform for potential antiviral development.

IMPORTANCE

Human metapneumovirus (HMPV) is a paramyxovirus that causes respiratory disease worldwide. It has been previously shown that HMPV requires binding to heparan sulfate on the surfaces of target cells for attachment and infection. In this study, we characterize the key features of this binding interaction using heparan sulfate mimetics, identify an important sulfate modification, and demonstrate that these interactions occur at the apical surface of polarized airway tissues. These findings provide insights into the initial binding step of HMPV infection that has potential for antiviral development.

Engineering, Structure and Immunogenicity of the Human Metapneumovirus F Protein in the Postfusion Conformation

Human metapneumovirus (hMPV) is a paramyxovirus that is a common cause of bronchiolitis and pneumonia in children less than five years of age. The hMPV fusion (F) glycoprotein is the primary target of neutralizing antibodies and is thus a critical vaccine antigen. To facilitate structure-based vaccine design, we stabilized the ectodomain of the hMPV F protein in the postfusion conformation and determined its structure to a resolution of 3.3 Å by X-ray crystallography. The structure resembles an elongated cone and is very similar to the postfusion F protein from the related human respiratory syncytial virus (hRSV). In contrast, significant differences were apparent with the postfusion F proteins from other paramyxoviruses, such as human parainfluenza type 3 (hPIV3) and Newcastle disease virus (NDV). The high similarity of hMPV and hRSV postfusion F in two antigenic sites targeted by neutralizing antibodies prompted us to test for antibody cross-reactivity. The widely used monoclonal antibody 101F, which binds to antigenic site IV of hRSV F, was found to cross-react with hMPV postfusion F and neutralize both hRSV and hMPV. Despite the cross-reactivity of 101F and the reported cross-reactivity of two other antibodies, 54G10 and MPE8, we found no detectable cross-reactivity in the polyclonal antibody responses raised in mice against the postfusion forms of either hMPV or hRSV F. The postfusion-stabilized hMPV F protein did, however, elicit high titers of hMPV-neutralizing activity, suggesting that it could serve as an effective subunit vaccine. Structural insights from these studies should be useful for designing novel immunogens able to induce wider cross-reactive antibody responses.

DC-SIGN and L-SIGN Are Attachment Factors That Promote Infection of Target Cells by Human Metapneumovirus in the Presence or Absence of Cellular Glycosaminoglycans

It is well established that glycosaminoglycans (GAGs) function as attachment factors for human metapneumovirus (HMPV), concentrating virions at the cell surface to promote interaction with other receptors for virus entry and infection. There is increasing evidence to suggest that multiple receptors may exhibit the capacity to promote infectious entry of HMPV into host cells; however, definitive identification of specific transmembrane receptors for HMPV attachment and entry is complicated by the widespread expression of cell surface GAGs. pgsA745 Chinese hamster ovary (CHO) cells are deficient in the expression of cell surface GAGs and resistant to HMPV infection. Here, we demonstrate that the expression of the Ca(2+)-dependent C-type lectin receptor (CLR) DC-SIGN (CD209L) or L-SIGN (CD209L) rendered pgsA745 cells permissive to HMPV infection. Unlike infection of parental CHO cells, HMPV infection of pgsA745 cells expressing DC-SIGN or L-SIGN was dynamin dependent and inhibited by mannan but not by pretreatment with bacterial heparinase. Parental CHO cells expressing DC-SIGN/L-SIGN also showed enhanced susceptibility to dynamin-dependent HMPV infection, confirming that CLRs can promote HMPV infection in the presence or absence of GAGs. Comparison of pgsA745 cells expressing wild-type and endocytosis-defective mutants of DC-SIGN/L-SIGN indicated that the endocytic function of CLRs was not essential but could contribute to HMPV infection of GAG-deficient cells. Together, these studies confirm a role for CLRs as attachment factors and entry receptors for HMPV infection. Moreover, they define an experimental system that can be exploited to identify transmembrane receptors and entry pathways where permissivity to HMPV infection can be rescued following the expression of a single cell surface receptor.

IMPORTANCE

On the surface of CHO cells, glycosaminoglycans (GAGs) function as the major attachment factor for human metapneumoviruses (HMPV), promoting dynamin-independent infection. Consistent with this, GAG-deficient pgaA745 CHO cells are resistant to HMPV. However, expression of DC-SIGN or L-SIGN rendered pgsA745 cells permissive to dynamin-dependent infection by HMPV, although the endocytic function of DC-SIGN/L-SIGN was not essential for, but could contribute to, enhanced infection. These studies provide direct evidence implicating DC-SIGN/L-SIGN as an alternate attachment factor for HMPV attachment, promoting dynamin-dependent infection via other unknown receptors in the absence of GAGs. Moreover, we describe a unique experimental system for the assessment of putative attachment and entry receptors for HMPV.

Human metapneumovirus uses endocytosis pathway for host cell entry

Human metapneumovirus (hMPV) is a prevalent pathogen worldwide and causes various respiratory infections. Although it is a critical pathogen in pediatric patients, it is unclear how it enters host cells. In this study, we focused on hMPV cell entry using two kinds of cell lines (Vero E6 and LLC-MK2), which are most commonly used for isolating and propagating for hMPV, and we used fluorescent dyes to label the virus particles and monitored how they enter the host cell in real time. We found that endocytosis was the predominant pathway by which hMPV entered host cells. When the virus particles were traced inside host cells, we found that a low intracellular pH was needed for intracellular fusion in LLC-MK2 cells.

Fusion of Enveloped Viruses in Endosomes

Ari Helenius launched the field of enveloped virus fusion in endosomes with a seminal paper in the Journal of Cell Biology in 1980. In the intervening years, a great deal has been learned about the structures and mechanisms of viral membrane fusion proteins as well as about the endosomes in which different enveloped viruses fuse and the endosomal cues that trigger fusion. We now recognize three classes of viral membrane fusion proteins based on structural criteria and four mechanisms of fusion triggering. After reviewing general features of viral membrane fusion proteins and viral fusion in endosomes, we delve into three characterized mechanisms for viral fusion triggering in endosomes: by low pH, by receptor binding plus low pH and by receptor binding plus the action of a protease. We end with a discussion of viruses that may employ novel endosomal fusion‐triggering mechanisms. A key take‐home message is that enveloped viruses that enter cells by fusing in endosomes traverse the endocytic pathway until they reach an endosome that has all of the environmental conditions (pH, proteases, ions, intracellular receptors and lipid composition) to (if needed) prime and (in all cases) trigger the fusion protein and to support membrane fusion.